Westersund
Phototaxis and a tidal rhythm
Abstract
Under high intensity fluorescent light, the intertidal
chiton, Mopalia muscosa was shown to be negatively phototactic
when left 43 to 27 hours. Chitons seemed to respond to tide both
exogenously and endogenously by reduction of movement. The
suggested tidal rhythm appears to be the mechanism by which these
chitons may predict the heighths of low tides. The rhythm appears
to have been rephased by a single artificial low tide of 10 to 45
minutes. Diurnal movement patterns appear to be exogenous.
Westersund, Page 2
Control of Movement in a Chiton
Introduction
The chiton Mopalia muscosa (Gould, 1846) is flexible in habit
and habitat. In the present study on the Monterey Peninsula, California.
M. muscosa was found at low tide (1) in permanent tide pools, (2) attached
to the undersides of large rocks in sand with Mopalia lignosa (Gould, 1846).
(3) in the midst of tight clusters of the sea anemone, Anthopleura
elegantissima (Brandt, 1835), (4) in the shade of various macroscopic
algae, (5) in bare rock crevices with only microscopic and crustose
algae, and (6) on exposed surfaces where their shells are often bleached
white by the sun. Barnawell (1954) notes that M. muscosa is one of the
few species of Mopalia that can do well in the estuarine conditions of
San Francisco Bay. This variety of habitats implies behavioral flexi-
bility as well as wide physiological tolerances.
Field observations by Smith (1975), myself, and others indicate
that Mopalia muscosa moves mostly at high tide and at night. In an
observation which led to the present research, chitons in a high tide
pool began to move at sunset. At the same time, a rising tide splashed
the tide pool with waves, and the water temperature dropped rapidly from
24°C. to 15°C. The chitons may have begun to move in response to the
onset of darkness, the disturbance of the pool by wave splash, the lowered
temperature, or an endogenous cycle. A series of experiments was planned
to determine the effect of each stimulus isolated from the rest. All work
was carried out at the Hopkins Marine Station of Stanford University during
the period April-June, 1974.
Westersund, Page 3
Control of Movement in a Chiton
Effects of Light, Turbulence, Temperature
and Water Level on Movement
Methods. Five aquaria, each containing 5 Mopalia muscosa, were
placed in darkrooms equipped with running sea water. In each tank, 3
of the chitons were freshly collected, and the other 2 had been in the
laboratory 1 to 2 weeks. Individual animals were not marked, but were
recognized'by size and adhering algal cover. Each aquarium was held
under a different set of test conditions for a period of 12 hours, as
shown in Fig. 1, and explained below. Tanks A, B, and D were held in
the same darkroom, while Tanks C and E were isolated in separate dark-
rooms.
At the start of the experiment and every 30 minutes thereafter,
the position of the center or midpoint of the animal was established
to the nearest centimeter. The straight-line distance between consecu¬
tive midpoint positions was calculated and recorded as the distance
moved (net displacement) per hour. Observations under dark conditions
were made with a flashlight covered with a red filter, to which the
chitons appeared to give no response.
Results and Discussion. The control animals in Tank A (Fig. 1-A)
held at constant darkness, temperature, and turbulence, showed little
movement. Small peaks of activity were noted which (unfortunately)
occurred when conditions were changed in other tanks. Since the control
was held in the same room with Tanks B and D, the manipulations carried
out on these other tanks may have altered the noise level or some other
Westersund, Page 4
Control of Movement in a Chiton
factor in the room to cause these small activity peaks. It is also
possible that these were random fluctuations.
The animals in Tank B, subjected to varying turbulence (Fig. 1-B)
showed no obvious response to this gentle stimulus.
In Tank C, exposed to alternate periods of light and darkness
(Fig. 7-C), there was less movement during light than during dark
periods; and the first onset of a dark period was accompanied by a
sharp increase in activity.
In Tank D, (Fig. ]-D) the animals responded to a falling water
level by moving down the sides of the tank, keeping half susmerged until
they reached the bottom. Although the aquarium bottom remained wet
during the artificial low tides, the chitons moved very little until
the "tide" rose again.
Animals kept in Tank E with a temperature cycle (Fig. 1-E) showed
increased activity at the onset of each change.
The results indicate that movement may be stimulated by decreases
in incident light, or by a change in water temperature or water level.
Endogenous Rhythms
4lthough the results obtained above indicate that movement may
be stimulated by some exogenous factors; thepossibilityofanexogenous
rhythm remains unerplored.—To examine-this-questien, the following
experiment was perfored  e  veve passe egeo
phytn
Westersund, Page 5
Control of Movement in a Chiton
Methods. Eight chitons from low tide pools and 8 from other
habitats were collected and placed in aquaria equipped with running sea
water at 14°C. in constant darkness. Animals ranged in length from 5 to
9 cm. Subsequet Observations were made with the aid of a dim red light.
The position of the midpoint of each chiton was checked every 45 minutes
.
over a 25 hour period;after the chitons had been in the aquaria .5, 3, 13
and 19 days. Movement (net displacement) was recorded as in the previous
experiment.
Results and Discussion. Fig. 2-A shows the total activity of
all 16 animals for a 25 hour period starting 12 hours after they were
placed in aquaria. Minimum activity, corresponding to actual lower low
tide, is followed immediately by an increase in activity. After 3 days
in the tank (Fig. 2-B), a sharp increase in activity of the animals
still corresponds to the rising tide following the lower low tide, but
there is less overall activity. After 13 days (Fig. 2-C) activity is
again low during the lower low tide and the overall activity is again
high. No tidal cycle is clearly resolvable after 19 days in the tanks
(Fig. 2-D). The results obtained are very suggestive of a tidal rhythm
of movement. The lull in activity at low tide periods coincides with
the field observations of Smith (1975) who found that Mopalia muscosa
that were uncovered by the tide had already "homed", while other animals
slightly lower in the intertidal that were not going to be exposed did
not home, but did reduce their movement. Activity was greatest during
C
Westersund, Page 6
Control of Movement in a Chiton
spring tides, days  and 13. These results eombined suggest the animals
may predict monthly fluctuations of tidal height. Further studies are
needed to confirm and expand on these preliminary conclusions.
Westersund, Page 7
Control of Movement in a Chiton
Summary
(1) Mopalia muscosa appears to be exogenously stimulated to move
by decreases in incident light, or by a change in water temperature or
water level.
(2) M. muscosa responds to an artificially lowered water level
by moving down, staying half submerged. The chitons move very little
in an empty tank until it is refilled.
(3) M. muscosa shows evidence of an endogenous tidal rhythm in
their movement. Under constant aquarium conditions, they move less at
times corresponding to low tide, especially the lower low tide, and
they move much more soon after.
(4) M. muscosa under constant aquarium conditions shows more
overall movement and clearer tidal cycles during spring tides. The
same monthly cyclé may enable them to predict the heights of low tides.
(5) No endogenous diurnal activity pattern was detected.
Westersund, Page 8
Control of Movement in a Chiton
Acknowledgments
I would like to thank and acknowledge the help of: Dr. Robin
Burnett, who advised me during this study and the writing of this paper:
David W. Phillips, who gave advice and loaned equipment; Dr. W. G. Evans.
who advised and encouraged me to study tidal rhythms; and Margaret
Westersund, my wife, who worked with me observing chitons at all hours.
2
Westersund, Page 9
Control of Movement in a Chiton
Literature Cited
Barnawell, Earl Baker
1954. The biology of the genus Mopalia in San Francisco Bay.
M.A. Thesis. Dept. of Zoology, University of California, Berkeley.
85 pp.; 9 figs..
Suanne Yvonne
Smith,
1975. Temporal and spatial activity patterns for the intertidal
The pellshe.
chiton Mopalia muscosa. The Veliger.
sine tastous
Westersund, Page 10
Control of Movement in a Chiton
Figure Captions
es haone en )
Fig. 1. Total distance moved vs. time for 5 chitons in each of
5 aquaria held under conditions explained below. At the top of the
teth verte be eregene ern eirne an
figure is a curve representing the height of the tide and shading
e one o
revet,
representing night. In the activity curves, the finer lines show general
trends by connecting averages of adjacent data points. A. In this the
control, nothing was varied. Sea water circulated at 14°C., the tank
was kept full, and the room was kept dark. These were also the conditions
in the other tanks, except for those that were varied, as explained below
and shown above each of the other activity graphs. B. Tha gently turbulent
flow of sea water was stopped or restarted every 3 hours. Temperature
change was minimal. C. Cool bright overhead flourescent lights were
turned on or off every 3 hours. D. The tank was alternately drained or
refilled every 3 hours. The tank took about 1 hour to refill and 1.5 hours
to drain. During the 3 hour filling period, the water was kept flowing,
but during the draining period, the water was shut off. Therefore a small
temperature cycle probably accompanied the "tidal" cycle. E. The incoming
stream of water was changed from a source at 14°C. to a source at 25°C. or
back again every 3 hours. After 1.5 hours, the temperature of the tank
was within 1"C. of the incoming water temperature.
21 hoes forrl ne
Fig. 2. Total distance moved vs. time for 16 chitons maintained
in constant dark and observed after .5 (A), 3 (B), 13 (C), and 19 (D) days
in aquaria. External conditions of light and tide are shown for each
Westersund, Page 11
Control of Movement in a Chiton
conseqversd vevfu at bas
observation period. Chitons were observed every 45 minutes; As in Fig.
finer lines show general trends. Note the recurring pattern of reduced
activity at low tides followed by sharply increasing activity soon after-
ward.
K. UESTERSUND
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